1. Field of the Invention
The present invention relates to semiconductor devices suitable for ferroelectric memories and the fabricating methods of the same.
2. Description of the Related Art
In recent years, attentions have been focused on utilization of a ferroelectric film as a dielectric film of a capacitor. The capacitor is referred to as a ferroelectric capacitor, and a ferroelectric memory (FeRAM: Ferro-electric Random Access Memory) including the ferroelectric capacitor is a nonvolatile memory. The ferroelectric memory has advantages such as the capability of high-speed operations, low electric power consumption, excellent writing/reading durability and, therefore, it is expected to be further advanced in the future.
However, the ferroelectric capacitor has the characteristics of being prone to degradation of characteristics due to hydrogen gas and moisture from outside. For example, it is known that, in the case where a ferroelectric capacitor is constituted by a bottom electrode made from a Pt film, a ferroelectric film made from a PbZr1-xTixO3 film (PZT film) and a top electrode made from a Pt film which are laminated in order, when a substrate thereof is heated to a temperature of about 200° C. in an atmosphere of hydrogen partial pressure of about 40 Pa (0.3 Torr), the PZT film loses most of its ferroelectric characteristics. Further, it is known that, when heat treatment is applied to a ferroelectric capacitor which has absorbed moisture or exists near moisture, the ferroelectricity of the ferroelectric film of the ferroelectric capacitor is significantly degraded.
Because of such characteristics of the ferroelectric capacitor, low-temperature processes capable of suppressing generation of moisture to a maximum extent are selected as processes after the formation of the ferroelectric film, in the fabrication processes for a ferroelectric memory. Further, as the process for depositing an interlayer insulation film, a deposition process with a CVD (chemical vapor deposition) method or the like, using material gases which generate relatively little hydrogen is selected. Further, as techniques for preventing degradation of a ferroelectric film due to hydrogen and moisture, there have been suggested techniques which form an aluminum oxide film over the ferroelectric capacitor and techniques which form an aluminum oxide film on an interlayer insulation film formed on the ferroelectric capacitor and the like. This is because the aluminum oxide film has a function of preventing diffusion of hydrogen and moisture. Therefore, these techniques can prevent hydrogen and moisture from reaching the ferroelectric films, thus preventing degradation of the ferroelectric films due to hydrogen and moisture.
These techniques are described, for example, in Patent Documents 1 to 5.
After the formation of a ferroelectric capacitor, Al wirings are formed and further an oxide film is formed as an interlayer insulation film by a plasma CVD method or the like. During the formation of the oxide film, the ferroelectric capacitor may be also degraded. Therefore, before the formation of the interlayer insulation film, an aluminum oxide film covering the Al wirings is formed.
However, the aluminum oxide film is difficult to etch. In order to ensure conduction between a wiring formed above and the Al wiring, it is necessary to etch the aluminum oxide film to form a via hole. However, such a process is difficult. Consequently, the diameter of the via hole may become smaller than a designed value, or a tungsten plug to be formed as a via plug may become abnormal due to influences of deposited substances during etching, thereby resulting in poor contact. This reduces the design margin, increases the difficulty of providing stable characteristics, and reduces the yield. Therefore, currently, the total thickness of the aluminum oxide film is set to 70 nm or less.
[Patent Document 1] Japanese Patent Application Laid-open No. 2003-197878
[Patent Document 2] Japanese Patent Application Laid-open No. 2001-68639
[Patent Document 3] Japanese Patent Application Laid-open No. 2003-174145
[Patent Document 4] Japanese Patent Application Laid-open No. 2002-176149
[Patent Document 5] Japanese Patent Application Laid-open No. 2003-100994
[Patent Document 6] Japanese Patent Application Laid-open No. 2003-289074